The present invention relates to an internal combustion engine with a supercharger disposed in an intake system and more particularly to a supercharged internal combustion engine of a reciprocating piston type having an improved piston crank mechanism which can optimize the piston speed when the engine is in a supercharged condition and can vary the compression ratio in accordance with an operating condition of the engine.
An example of a supercharged internal combustion engine of a reciprocating piston type having a variable compression ratio mechanism is disclosed in Japanese Patent Provisional Publication No. 62-78440. It is disclosed in the publication to make lower the compression ratio at high load operation where supercharging is carried out, for thereby avoiding knocking, and make higher the compression ratio at low to middle load operation where supercharging is not carried out, for thereby attaining a good fuel consumption. The variable compression ratio mechanism variably controls the compression ratio through a variable control of the volume of a chamber in communication with an engine cylinder, which is attained by varying a position of a piston disposed in the chamber.
Generally, at high load operation where a large amount of air-fuel mixture is to be combusted, the burn duration tends to become longer. This tendency is enhanced when supercharging is carried out at high load operation, resulting in a problem that the exhaust gas temperature at high load operation becomes very high.
When the burn duration becomes longer, the combustion is not completed within a crank angle range (the first half of the expansion stroke) where the heat of the combustion can be effectively converted to the output of the engine. Accordingly, the heat generated at the latter period of the combustion is not effectively converted to the output of the engine but is used only for increasing the temperature of the exhaust gas, thus lowering the thermal efficiency of the engine and causing a high exhaust gas temperature at high load.
For this reason, in an internal combustion engine with a supercharger, it is required that a material having a high heat resistance be used for the parts around the combustion chamber and the parts of the exhaust system or the amount of fuel be increased considerably at high load where the engine is operated under a highly or sufficiently supercharged condition, for thereby lowering the exhaust gas temperature.
It is accordingly an object of the present invention to provide an internal combustion engine equipped with a supercharger, which is free from the above noted problems.
It is a further object of the present invention to provide an internal combustion engine of the foregoing character which can shorten the burn duration at high load operation, thereby prevent a rise of the exhaust temperature and improve the thermal efficiency of the engine.
It is a further object of the present invention to provide an internal combustion engine of the foregoing character which can variably control the compression ratio in accordance with a supercharging pressure, thereby prevent knocking when supercharging pressure is high and improve the fuel consumption when supercharging is not carried out.
To accomplish the above objects, the present invention provides an internal combustion engine comprising a piston reciprocatively movable within a cylinder of the engine, a piston crank mechanism for converting reciprocative motion of the piston to rotation of a crank shaft, and a supercharger for supercharging the cylinder, wherein the piston crank mechanism connects between the piston and the crankshaft so as to cause the piston to move at a speed which is lower around a top dead center of the piston and higher around a bottom dead center of the piston as compared with respective corresponding speeds attained by a comparable single-link type piston crank mechanism.